This invention relates to devices for generating, detecting, and analyzing surface acoustic waves.
A surface acoustic wave is one of several types of wave motion in which acoustic energy may appear as it travels through a solid medium. Bulk acoustic waves, for example, propagate through the interior of an acoustically conductive medium and, in general, exhibit a single type of motion, such as shear or longitudinal waves, while surface waves are a more complex mixture of these shear and longitudinal motions which is made possible by the presence of a stress free boundary condition. Because of this boundary condition, surface wave energy extends only a few wavelengths into the bulk of the propagating medium. This concentration of surface wave energy near the surface makes a variety of applications for surface waves possible in the field of electronics, such as signal filtering, the amplification of weak signals, the storage of signals in delay lines, the provision of highly accurate frequency references, and the detection of physical changes, like pressure or temperature, which affect the propagation characteristics of surface acoustic waves in a medium.
Practical applications for surface acoustic wave devices have expanded dramatically since the development of the interdigital transducer, which is capable of efficiently converting an electrical signal into a surface acoustic wave and vice versa. A simple interdigital transducer consists of a pair of interleaved electrodes which are placed in electrical contact with a piezoelectric material. When such a material is distorted, it produces an internal electric field. Conversely, if an electric field is applied to a piezoelectric material, the material will expand or contract, depending upon the polarity of the applied field. Because of this phenomenon, when a rapidly changing electrical signal is applied to a piezoelectric material through an interdigital transducer, the material will vibrate in response to the electrical signal, thereby generating a surface acoustic wave. A pair of single-fingered electrodes will not produce surface acoustic waves efficiently, but a multiple number of electrode fingers, when placed in an intedigitating pattern, will each excite an acoustic wave and, if the spacing between the fingers is properly related to the desired acoustic wavelength, the separately excited waves can be made to reinforce one another and produce a suitably large acoustic singal.
One of the most promising applications for surface acoustic wave technology has occurred in the design of crystal resonators for use in such devices as multipole crystal filters, crystal controlled oscillators, and tuned radio frequency receivers. In the past, bulk wave quartz crystal resonators with a high Q (quality factor) have been extensively employed to stabilize the frequencies of such oscillator circuits. Undesirable spurious modes can appear in the response of a bulk crystal resonator, however, and, in addition, bulk wave resonator technology is limited in its frequency range because a minimum thickness must be maintained in the crystal to ensure an adequate amount of physical strength. With the advent of surface acoustic wave techniques, a variety of improved crystal controlled oscillator designs have become feasible and offer an alternative approach which eliminates some of the problems which have been experienced with bulk wave devices.
One of the simplest of these alternative approaches utilizes a surface acoustic wave delay line, the output of which is fed back to the input through an amplifier which supplies excess gain. Acoustic waves are well suited for employment in a delay line, because their velocity is typically five orders of magnitude lower than the velocity of an electrical signal, permitting usefully long delay times to be obtained in a reasonably sized device having dimensions on the order of centimeters. The operating principle of the surface acoustic wave (SAW) delay line oscillator is based on forming a return loop with gain in which the phase shift around the loop is an integer multiple of 2.pi. radians for a particular frequency. To complete the device, this circuit must be coupled to a transmitting or receiving network, which may be electrical or, in the case of an acoustic device, may utilize a piezoelectric interaction.
Another type of SAW oscillator employs a pair of quarter wavelength spaced grating reflectors which are positioned on a substrate to form a surface wave resonant cavity. An interdigital transducer is placed within the cavity to provide the electrical input and output coupling port for the oscillator. Because of the narrow bandwidth associated with such distributed grating reflectors, it is possible to design a SAW resonator cavity which effectively responds to only a single longitudinal mode of acoustic waves.
The frequency control capabilities of the single mode SAW resonator are well known. In addition to this category of applications, however, the SAW resonator and the SAW delay line may also be used as strain sensors. In this configuration, an external perturbation, such as an acceleration or the weight of an object, is applied to distort the propagating medium. This perturbation is sensed by measuring the resulting change in the resonant frequency or the phase of the surface acoustic wave travelling in the delay line or the resonator. For the frequencies at which such devices are typically operated, the necessary electronics must be located physically close to the SAW components, since any significant length of connecting cable would introduce phase variations and adversely affect the measurement. In some applications for these devices, however, the environment in which the SAW sensor must be placed is too hostile (as in a high temperature, for example) for the electronic components to survive. Therefore, a need has developed in the SAW sensor art for a device in which the SAW components can be isolated from the electronics components.